Datasheet

V
OUT
=
-R
F
1 + sC
F
R
F
x
I
DIODE
1 +
¨
¨
©
§
C
F
+ C
IN
C
F
¨
¨
©
§
2
= 8S
2
´
¶
2
GBW
R
F
2
C
F
2
=
1
Z
C
F
R
F
LMP7717, LMP7718
www.ti.com SNOSAY7H –MARCH 2007–REVISED MARCH 2013
For the above equation s = jω. To find the actual amplitude of the equation the square root of the square of the
real and imaginary parts are calculated. At the intersection of F and A, we have:
(17)
After a bit of algebraic manipulation the above equation reduces to:
(18)
In the above equation the only unknown is C
F
. In trying to solve this equation the fourth power of C
F
must be
dealt with. An excel spread sheet with this equation can be used and all the known values entered. Then through
iteration, the value of C
F
when both sides are equal will be found. That is the correct value for C
F
and of course
the closest standard value is used for C
F
.
Before moving to the lab, the transfer function of the transimpedance amplifier must be found and the units must
be in Ohms.
(19)
The LMP7717 was evaluated for R
F
= 10 kΩ and 100 kΩ, representing a somewhat lower gain configuration and
with the 100 kΩ feedback resistor a fairly high gain configuration. The R
F
= 10 kΩ is covered first. Looking at the
Input Common Mode Capacitance vs. V
CM
chart for C
CM
for the operating point selected C
CM
= 15 pF. Note that
for split supplies V
CM
= 2.5V, C
IN
= 22 pF and f
GBW
= 88 MHz. Solving for C
F
the calculated value is 1.75 pF, so
1.8 pF is selected for use. Checking the frequency of the pole finds that it is at 8.8 MHz, which is right at the
minimum gain recommended for this part. Some over compensation was necessary for stability and the final
selected value for C
F
is 2.7 pF. This moves the pole to 5.9 MHz. Figure 63 and Figure 64 show the rise and fall
times obtained in the lab with a 1V output swing. The laser diode was difficult to drive due to thermal effects
making the starting and ending point of the pulse quite different, therefore the two separate scope pictures.
Figure 63. Fall Time
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